Regulation of nitrogen fixation and assimilation genes in the free-living versus symbiotic state

Abstract

Biological nitrogen fixation is an extremely energy intensive process. Under ideal conditions the reduction of one molecule of dinitrogen (N2), catalysed by the nitrogenase enzyme complex, requires 16 molecules of ATP. However, in vivo ATP requirements of up to 42 per N2 reduced have been measured (80; see 66). It is therefore not surprising that free-living nitrogen-fixing bacteria, such as Klebsiella pneumoniae, only reduce N2 when nitrogen (N-) starved and tightly regulate their nitrogen-fixation (nif) genes in response to the intracellular concentration of combined nitrogen (N-regulation). In the case of symbiotic nitrogen fixing bacteria such as rhizobia, the situation is different. As docile endosymbionts in nodules, having differentiated into a clearly different physiological state (bacteroids), they are provided by the plant host with (ample) carbon (C-) source(s) for the production of ATP and reducing equivalents, to facilitate nitrogen fixation for the plant’s benefit (see 66). Therefore, the nif/fix genes of these bacteria may not need to be subject to N-regulation.